STORY: London sizzled in the heatwave on Monday (July 18) with Britain on course for its hottest day on record.
Temperatures were forecast to hit 104 Farenheit (40 Celsius) for the first time, prompting t he government to trigger a "national emergency alert".
The Met Office said there would need to be "substantial" changes in working practices and daily routines.
And that there was a high risk of failure of heat-sensitive systems and equipment.
That could lead to localized loss of power, water or mobile phone services.
British Minister in Charge of Government Coordination, Kit Malthouse:
"So, you know, we will learn over the next 48 hours about how the rail system copes with this kind of heat, which it wasn't built to cope with. We will learn about how we deal as a community with heat. We'll learn about how things, you know, go well and don't go well over the next 48 hours. And then we'll have to do work over the next three, four, five years to cope with it. You know, this is a record we think is going to be achieved in the next 48 hours. It's also a moment for us to come together, get through it in good shape, and then think about what steps we need to take to deal with it if it occurs in the future."
London's metro network urged commuters to only travel if essential.
It imposed temporary speed restrictions, causing reduced services with journeys taking longer than normal.
The national rail network also urged passengers to stay at home.
It said some services - including a key route between northeastern England and London - would not run during parts of Tuesday (July 19).
The government urged schools to stay open - but some closed, resorting to lockdown-style online learning.
And others were due to close earlier than usual. Normal uniform demands were ditched and end-of-term sports days canceled.
Some factories brought their opening hours forward, to protect workers on the hottest jobs, such as welding.
At least one major zoo, at Chester, said it would close for two days, while London Zoo and Whipsnade Zoo said many animals would be able to retreat to "cool zones", while some exhibits might be closed.
Penguins at the London Aquarium kept as cool as they could thanks to frozen fish treats and ice-filled buckets.